1914] on Improvements in Long-Distance Telephony 105 



speech cannot be transmitted if the total attenuation of a hne 

 exceeds 4*5 or 4*8. 



For good speech it should not exceed 2*5 or 3*5. 



The extreme limit corresponds to about 47 miles of standard 

 telephone cable. It may be remarked, however, that the total 

 attenuation is not the only criterion of good speech transmission, 

 although it is an important one. 



In the case of the Anglo-French loaded (1910) cable the total 

 attenuation for that cable alone is only about 0*33. Hence, as 

 pointed out by Major O'Meara in his valuable paper before the 

 Institution of Electrical Engineers when describing the structure 

 and laying of this cable,* if this cable were continued by heavy 

 overland lines of copper wire weighing 800 lb. to the mile, speech 

 would be possible between London and Astrakhan on the Persian 

 Gulf. Accordingly, if a cable of this type about 200 miles in length 

 were laid under sea, speech would certainly be possible through it. 

 Good speech would be obtained through about 150 miles. 



Unless therefore a still greater reduction in attenuation constant 

 is practicable, we are a long way yet from telephony conducted 

 through an Atlantic cable or even telephony through a cable 

 1000 miles in length. 



As regards underground cables or aerial lines the case is very 

 different. It is possible to lay underground dry core or paper 

 insulated cables sheathed in lead, and to load them as heavily as 

 required by coils at intervals of a few miles. Also we are not so 

 much limited in the weight of the copper wdre per mile as in the 

 case of submarine cables, whilst the coils can be of any reasonable 

 size. 



• It is in this manner that great improvements in the speech trans- 

 mitting power of overhead and underground lines are being made 

 by our British General Post Office and by the postal and telegraphic 

 authorities of other countries, such as France, Germany, Italy, and 

 the United States, by suitaljle loading. 



The longest loaded aerial telephone line at present in use is that 

 from Xew York to Denver, Colorado, U.S.A., a distance of over 

 2000 miles. Before the introduction of loaded lines this range was 

 beyond economical telephonic transmission. This loaded line is an 

 aerial hne constructed of a copper wire No. 8 B.W.G., weighing 435 lb. 

 to the mile. Similar lines connect all the large cities in the Eastern 

 States, The circuits are complete or double metallic circuits, the 

 hues being twisted to prevent cross talk. They are associated in 

 pairs so that phantom circuits can be formed. The constants of each 

 circuit when unloaded are 4 • 14 ohms, ' 0091 microfarad and ' 0034 

 henry, all per loop mile. They are loaded by inserting every 8 miles 



* See Major W. A. J. O'Meara, Submarine Cables for Long Distance 

 Telephone Circuits, Journ. Inst. Elect. Eng., vol. xlvii., p. 309, 1911. 



